Involvement of the frontal ventrolateral orbital cortex in descending inhibition of nociception mediated by the periaqueductal gray in rats.

Our previous findings which indicated that electrical stimulation of ventrolateral orbital cortex (VLO) can depress the rat tail flick (TF) reflex and that the VLO-evoked inhibitory effect is blocked by electrolytic lesions of periaqueductal gray (PAG) suggest a role of the VLO in the modulation of nociception. To further investigate the involvement of VLO in this nociceptive modulatory pathway, we tested the effects of microinjections of glutamate (200 mM, 0.7 microliter) into the VLO on the TF reflex. An unilateral microinjection of glutamate into the VLO significantly depressed the TF reflex; and this effect was repeatable. Furthermore, bilateral microinjections of gamma-aminobutyric acid (GABA: 100 mM, 0.5 microliter on each side) into the ventrolateral parts of PAG could eliminate this VLO-evoked inhibition of the TF reflex. These results, along with our previous findings provide further support for a hypothesis that VLO, as a higher center in the frontal cortex, plays an important role in modulation of nociception, and this role is mediated by PAG leading to activation of the brainstem descending inhibitory system which depresses the nociceptive information at the spinal level.